IfChanged - returns 0 if file changed, 127 otherwise

ifchanged.sh

#! /bin/sh

# IfChanged - returns 0 if file changed, 127 otherwise
# Copyright (C) 2013 Matous J. Fialka, <http://mjf.cz/>
# Released under the terms of The MIT License

fpath="$(realpath 2>/dev/null ""$1"")" || {
	echo "Error: realpath $1 failed" 1>&2
	exit 1
}

dpath="$HOME/.ifchanged/${fpath%/*}"

mkdir -p "$dpath" &>/dev/null || {
	echo "Error: mkdir -p $dpath failed" 1>&2
	exit 2
}

if [ -e "$dpath/$1" ]
then
	mv "$dpath/$1" "$dpath/$1.last" &>/dev/null || {
		echo "Error: mv $dpath/$1 $dpath/$1.last failed" 1>&2
		exit 3
	}
else
	sha1sum "$fpath" 2>/dev/null 1>"$dpath/$1" || {
		echo "Error: sha1sum $fpath > $dpath/$1 failed" 1>&2
		exit 4
	}
fi

sha1sum "$fpath" 2>/dev/null 1>"$dpath/$1" || {
	echo "Error: sha1sum $fpath > $dpath/$1 failed" 1>&2
	exit 5
}

diff -q "$dpath/$1" "$dpath/$1.last" &>/dev/null || exit 0

exit 127

Use it as simple as follows:

:ifchanged test && echo CHANGED
Error: realpath test failed
:touch test
:ifchanged test && echo CHANGED
:ifchanged test && echo CHANGED
:echo TEST > test
:ifchanged test && echo CHANGED
CHANGED
:ifchanged test && echo CHANGED

Enjoy.